The long term objective of this research proposal is to elucidate the role of CYP2J2 in the QT- prolonging, cardiotoxic effect of certain xenobiotics. CYP2J2 is abundant in cardiac myocytes and is involved in the metabolism of arachidonic acid to biologically active epoxyeicosatrienoic acids (EETs). EETs have well documented electrophysiological effects on cardiac ion channels and the QT-period in various cell models. We have demonstrated that CYP2J2 is inhibited by 70% of drugs that are strongly associated with QT-prolongation. Therefore, alterations in tissue EET(s) concentrations caused by CYP2J2 inhibitors could be expected to have toxic effects on cardiomyocytes. This project is developed around two hypotheses and four specific aims. Hypothesis one states that drugs associated with torsade de pointes can be identified that strongly inhibit CYP2J2, but have minimal effect on hERG at physiological concentrations. Two specific aims were designed to test this hypothesis and are focused on characterizing the physiological effect of CYP2J2 inhibition by drugs that cause QT- prolongation in both a recombinant system and cardiac microsomes from transgenic mice that express CYP2J2 in their hearts. The second hypothesis states that drugs that inhibit CYP2J2 contribute to prolongation of the action potential in mouse cardiac myocytes by reducing EET levels, which in turn leads to reduction in KATP currents. To test the second hypothesis, specific aims 3 and 4 were developed to determine if chronic treatment of mice with potent CYP2J2 inhibitors, identified in specific aims 1 and 2, leads to reduction in endogenous EETs levels in mouse cardiac tissue and finally specific aim 4 utilizes electrophysiological studies in cardiac myocytes isolated from wild type and transgenic mice to determine the consequences of inhibiting CYP2J2 on KATP currents and the action potential duration. Successful completion of these aims will demonstrate, for the first time, the importance of metabolism in the mechanism of toxicity of torsadogenic drugs, by characterizing the functional interactions of a unique enzyme (CYP2J2) that is situated at the interface of drug and endogenous substrate metabolism.